1. Technical Field
The present invention relates to a method and an apparatus for supplying sealing air to an exhaust turbine, which interacts with an internal combustion engine for turbo-compound operation in a vehicle. The exhaust gases from the internal combustion engine being received in an exhaust system having a supercharger turbine that drives a compressor for the engine combustion air. Residual energy in the exhaust gas flow is recovered via the exhaust turbine for transfer to the crankshaft of the internal combustion engine. The exhaust turbine is supported in a bearing housing, which is fed with sealing air via a fluid line, and the exhaust system includes an exhaust brake throttle having an exhaust gas pressure regulator for regulating the exhaust brake pressure.
2. Background
In a turbo-compound engine, an exhaust turbine, for example an axial-flow turbine, is used to recover residual energy from the exhaust gases of an internal combustion engine after the exhaust gases have been used to drive a turbocompressor that compresses engine charge-air. The exhaust gases drive the exhaust turbine at a speed of up to approximately 90,000 revolutions per minute (rpm). In a bearing housing, seals are used on the exhaust turbine drive shaft in order to prevent lubricating oil from escaping and to prevent gases from leaking in. Due to the high operating speeds and temperatures, the seals are exposed to great stress.
A known method for improving the functioning of the exhaust turbine seals is to pressurize the bearing housing by way of a pressure line and a bore. In this manner, an overpressure in relation to the ambient pressure is maintained between two seals in the bearing housing. A certain proportion of this buffer pressure is allowed to pass through the outer seal out into the exhaust system and the remainder passes through the inner seal and reaches the inside of the crankcase.
Under normal operating conditions, a negative pressure prevails on the outlet side of the exhaust turbine rotor. This is due to the centrifugal effects (forces). On the other side of the seals, the crankcase pressure prevails (normally a slight overpressure in relation to the atmospheric pressure). In the absence of any buffer pressure, which amounts to approximately 0.5 bar gauge, the pressure differential might normally lead to an oil leakage.
On activation of an exhaust brake, which is located downstream of the exhaust turbine and comprises (includes, but is not limited to) a throttle for stopping the exhaust gas flow through exhaust ports of the internal combustion engine, the pressure upstream of the exhaust brake throttle can rise to approximately 5 bar gauge, and the gas temperature can reach approximately 700 degrees Celsius. This pressure and temperature rise means that when engine braking, the seals are instantaneously exposed to much greater stresses than under other operating conditions. As a result, very hot and not particularly clean gases can leak into the bearing housing if the overpressure therein is not increased to a correspondingly higher level. The hot gas may mean that the working temperature of the seals and the bearing are exceeded, which may have an adverse effect on the reliability of the system. In order to ensure that exhaust gases do not enter into the bearing housing, the buffer pressure in the bearing housing should therefore be kept somewhat higher than the pressure in the exhaust brake.
Accordingly, the buffer pressure in the bearing housing should therefore be between 0.5 and 6.0 bar gauge, depending on experienced operating conditions. Theoretically, it should be possible to maintain the higher pressure level at all times, but this leads to a high air consumption and the constant delivery of a high volumetric flow to the crankcase. This also means that no air source other than the mechanical compressor of the vehicle could be used in order to deliver the high pressure, which means that unacceptably high parasitic losses occur in the internal combustion engine.